Allaah has prescribed the best and most just of rulings, which bring happiness to individuals, families and societies. Hence He has prescribed rights and duties for every individual in society, so life will run smoothly and each person will know what he is entitled to and what he must do.
The husband’s rights over his wife are great, and have to do with him, his wealth, his house and his family. Showing respect to the husband’s family, especially his parents, is part of showing respect to one’s husband and treating him well. When a woman does her duties towards her husband, she is obeying her Lord and has the hope of reward from Him.
Honouring your mother-in-law is something that is to be appreciated, and no one can deny that. Rather what you are doing is in accordance with Islam and common sense. By doing that you are earning the pleasure of your husband and contributing to the success of your marriage and protecting it against things that may spoil it.
But one should not obey one's Lord in one way and forget other aspects. It is not permissible for your respect towards your mother-in-law to be at the expense of respect for your own mother, because parents – and especially the mother – have important rights over their children. See the answer to question no. 5053 for more information on the mother’s rights over her children.
You have to honour your mother and treat her kindly even if she is a kaafir, as much as you can and as much as your circumstances allow. But if she calls you to kufr and strives to make you follow that, as our Lord says (interpretation of the meaning):
“And We have enjoined on man to be good and dutiful to his parents; but if they strive to make you join with Me (in worship) anything (as a partner) of which you have no knowledge, then obey them not”
“But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not; but behave with them in the world kindly”
So what are her rights over you if she is a Muslim who believes in the Oneness of Allaah (Tawheed)?
It was narrated that Asma’ bint ‘Abi Bakr (may Allaah be pleased with her) said: My mother came to visit me when she was a mushrikah, at the time of the Messenger of Allaah (peace and blessings of Allaah be upon him). I consulted the Messenger of Allaah (peace and blessings of Allaah be upon him) and said: She is expecting (something), should I uphold the ties of kinship with my mother? He said: “Yes, uphold the ties of kinship with your mother.”
Narrated by al-Bukhaari (2477) and Muslim (1003).
What is meant by expecting something is that she wants her daughter to honour her.
So do two good things at the same time: honour your mother and your mother-in-law. If there is a conflict between the two, then do not give your mother-in-law or any other woman precedence over your mother, so long as that will not cause any trouble between you and your husband.
We ask Allaah to help you to do that which pleases Him and to make you among the woman who are righteous and devout.
And Allaah knows best.